The present invention is directed to providing an improved image raster expanding system for a television receiver.
Prior systems are known in which the central portion of the image may be enlarged by expanding the image raster in one or both directions, and if raster expansion is effected in both directions, it is usually done in such proportions as to maintain the aspect ratio unchanged. In the expanded mode the electron beam is deflected beyond the image display screen of the cathode ray picture tube. It is observed that there is a loss of contrast and brightness in the expanded image, that is the picture appears to be washed out. This is annoying to the viewer and requires him to readjust the television receiver. However, it usually is not possible even with readjustment to obtain equivalent picture brightness and contrast. Moreover, in a color television receiver, raster expansion usually entails a change in average color level resulting in undesirable color distortion in the enlarged image.
This invention overcomes the serious drawbacks just mentioned by a combination of two importance features. First it has been found that by blanking the electron beam during the interval when the electron beam would be deflected beyond the screen of the cathode ray picture tube, the accompanying deterioration in contrast and brightness levels of the image is greatly reduced. A unique circuit has been devised for providing both horizontal and vertical overscan blanking. Secondly, means are provided for varying the contrast and color level adjustments by an amount sufficient to substantially compensate changes in contrast and color level which accompany raster expansion. In the preferred embodiment, switch means are provided for actuating both the overscan blanking means and the contrast and color level varying means concomitantly with the raster expanding means. However, substantial benefits may be realized by switch-actuating either the overscan blanking means or the contrast and color level varying means alone.
As a result of these advances, a greatly improved image raster expanding system for a television receiver has been achieved. This feature provides a zoom effect by producing a blow up or enlargement of the central portion of the normal image which is substantially as bright and colorful as the original image.